Rewetting alongside biochar and sulphate addition mitigates greenhouse gas emissions and retain carbon in degraded upland peatlands

Peat soils store significant amounts of carbon (C) globally, and increased C sequestration into peatlands could play a role in offsetting anthropogenic greenhouse gas (GHG) emissions. As such, there is a need to find and assess optimal greenhouse gas removal (GGR) interventions to minimise GHG losses, protect current C stocks, and promote further C sequestration. This mesocosm study assessed the additional C storage potential of different C-rich substrates (Juncus straw or Juncus-derived biochar) and/or FeSO₄ amendments, with a low water table (LW; −15 cm) and high-water table (HW; 0 cm) in intact soil columns (20 cm diam. x 25 cm deep) extracted from degraded upland peat. GHG fluxes, soluble nutrients, changes in microbial community structure and enzyme activity were measured over a one-year period to evaluate the net C storage and their overall GGR potential. HW reduced CO₂ emissions by 28 % compared to LW, while CH₄ emissions increased, ultimately contributing 61 % of the overall CO₂ equivalent (CO₂eq) GHG emissions in HW cores with no amendments. Organic amendments had a significant effect on CO₂ and CH₄ emissions with the highest cumulative emissions being from the Straw-HW (26.2 t CO₂eq ha⁻¹ yr⁻¹) and the lowest cumulative emissions being from the Biochar + FeSO₄+HW (7.9 t CO₂eq ha⁻¹ yr⁻¹). Biochar + FeSO₄+HW led to the strongest net gain in soil C, suppressing decomposition of the native peat-C as well as CH₄ emissions. The application of FeSO₄ significantly reduced CO₂eq GHG emissions by preventing methanogenesis through alternative electron acceptors. The Biochar + HW treatment suppressed Ascomycota abundance by 22 %. The combination of a high-water level, biochar addition and availability of alternative electron acceptors (e.g., FeSO₄) can increase net C accumulation and GGR via both abiotic and biotic mechanisms, including i) increased C input, ii) modulation of soil microbiome to reduce peat turnover; and iii) suppression of CH₄ and N₂O emissions.